1. Field of the Invention
The present invention relates to a radio communication apparatus and, more particularly, to a mobile or portable radio communication apparatus which has a communication function capable of coping with a plurality of different communication systems.
2. Description of the Prior Art
Conventional portable telephones in a personal handy phone system (PHS) and a cellular system can perform communication only within the respective systems but cannot perform communication between different systems.
In each of these PHS and cellular systems, a plurality of radio base stations form communication zones, and a portable telephone communicates with a radio base station which forms the communication zone in which the portable telephone is present. The communication zones partially overlap each other. A portable telephone located at such an overlapping portion receives signals from a plurality of radio base stations, and can transmit signals to a plurality of radio base stations.
In such a case, to select one of the radio base stations, the portable telephone measures the electric field strengths or bit error rates of signals from the respective radio base stations and communicates with a radio base station exhibiting good radio channel quality.
For example, such radio communication apparatuses designed to select a radio base station in one communication system on the basis of the radio channel quality are disclosed in Japanese Unexamined Patent Publication Nos. 7-327253 and 6-292258.
Radio communication apparatuses capable of performing communication through a plurality of communication systems have recently been proposed. Such a radio communication apparatus must select a specific communication system through which communication is to be performed. For example, Japanese Unexamined Patent Publication No. 7-177570 discloses a radio telephone apparatus capable of performing communication through two communication systems based on the cellular mode and the PHP (Personal Handy Phone) mode. In a communication system selection method for this apparatus, to determine the presence/absence of the radio telephone apparatus in the service area of a PHP communication system in the initial stage of communication, the apparatus checks whether a signal from a PHP base station is received or not. More specifically, if the radio telephone apparatus has received a radio wave from the PHP base station, the radio telephone apparatus determines that the apparatus is present in the service area of the PHP communication system, and performs communication through the PHP communication system. If the radio telephone apparatus cannot receive a radio wave from the PHP base station, the apparatus determines that it is not present in the service area of the PHP communication system, and communication through the other communication system based on the cellular mode is executed. The possibility of communication through the PHP communication system is determined preferentially in this manner because the communication cost in the PHP communication system is lower than that in the other system, and the service area is small.
As described above, the radio telephone apparatus disclosed in Japanese Unexamined Patent Publication No. 7-177570 can automatically perform an originating operation to one of the systems by determining whether the apparatus is present in the service area of the PHP communication system, in consideration of the communication cost. Since this determination is performed by checking whether the apparatus is present in the service area of the PHP communication system, the PHP communication system is selected regardless of channel quality if the apparatus is present in the service area of the PHP communication system.
After a communication system is selected, a radio base station may be selected on the basis of radio channel quality, as described in Japanese Unexamined Patent Publication Nos. 7-327253 and 6-292258.
Assume that the channel quality provided by the cellular communication system is better than that provided by the PHP communication system at the time of channel connection. Even in this case, if the apparatus is present in an area where it can communicate through the PHP communication system, the PHP communication system, in which the communication cost is low, is automatically selected. Even if, therefore, a radio base station exhibiting good channel quality in the system is selected on the basis of the electric field strength or the like after a communication system is selected, this does not so improve the overall channel quality.